This invention relates to gamma ray scattering or gamma-gamma density well logging techniques and, more particularly, to such techniques for deriving a compensated formation bulk density by the use of a position sensitive scintillation detector.
It has become fairly common practice in the art of well logging to log the earth formations in the vicinity of a well borehole which is either cased or uncased with a gamma ray density instrument. Such an instrument comprises a source of gamma rays such as cesium 137 which are collimated and directed outwardly into the formation from the well logging sonde or tool which is lowered into the borehole on an electrical wireline. The gamma rays are scattered from the electrons of elements comprising the earth formations in the vicinity of the borehole. A separate gamma ray detector longitudinally spaced from the gamma ray source then is used to measure the intensity of scattered gamma rays from the materials surrounding the well borehole back into the instrument. More than one such detector may be used in order to provide a compensated density measurement which is compensated for the presence of mudcake or casing thickness intervening between the gamma ray source and the detector or detectors. An example of such a system is given in U.S. Pat. No. 4,297,575 which is assigned to the assignee of the present invention.
In the well logging system described in the aforementioned patent, the gamma ray source is positioned below two gamma ray detectors which are longitudinally spaced at different distances from the source. Gamma rays from the source are directed by a collimator into the earth formations in the vicinity of the wellbore. Scattered gamma rays returning from these formations are directed by collimators to the two detectors. In this system, the near spaced detector to the source is a Geiger-Mueller type counting tube and the far spaced detector is a sodium iodide thorium activated scintillation detector having a photomultiplier tube optically coupled thereto. The thickness of casing or mudcake may be determined by appropriate computations based on predetermined relationships existing between the count rate of scattered gamma rays in the short spaced detector to those in the long spaced detector. The count rate in the short spaced detector is much more influenced by material closer to the well borehole than that of the long spaced detector. By appropriately combining the count rates in the two detectors, which are spaced a known distance apart, and the use of a predetermined calibrated relationship between the formation bulk density and the count rates at the two detectors the formation density may be computed independently of the effects of the mudcake or casing intervening the distance between the scattered gamma rays from the source and the two detectors.
Certain problems are encountered in a system of the type described in the above referenced U.S. patent. These concern the sensitivity or relative sensitivity to gamma rays of the two different types of detectors and the spacing distance used to separate the detectors being related geometrically to the depth of investigation of the instrument into the earth formation surrounding the well borehole. It would be highly desirable to have a multiplicity of detectors located above the gamma ray source so that the formation bulk density at different radial distances from the well borehole could be investigated and a radial density cross-section of the earth formation in the vicinity of the borehole plotted as a function of borehole depth. It is a feature of the present invention that a single unique position sensitive scintillation detector is utilized together with a gamma ray source to provide measurements of formation bulk density at different radial distances from the well borehole and to compensate the measured formation densities for the effects of mudcake or intervening casing thickness and cement thicknesses between the well borehole and the surrounding earth formations. It is another feature of the present invention to be able to derive formation bulk density measurements at different radial distances from a well borehole and to plot such formation densities as a function of borehole depth. Yet another feature of the present invention comprises the ability to measure the scattered gamma ray energy dependent response from earth formations adjacent a well borehole at different radial distances from the well borehole with a single position sensitive radiation detector.